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Influence of the rotor nacelle assembly mass on the design of monopile foundations

M.L.A. Segeren, N.F.B. Diepeveen
Faculty of Civil Engineering and GeoSciences, Delft University of Technology, the Netherlands

In light of the developments of the offshore wind industry in terms of water depth and turbine size, the objective of the research presented in this paper is to gain insight in the applicability limits of the monopile support structure for large offshore wind turbines. This is done by demonstrating how the mass of the rotor nacelle assembly (RNA) of a turbine influences the design of monopile support structures. A fictitious 5MW class wind turbine with 126 m rotor diameter is used as reference case. The typical RNA mass of existing turbines in this class is around 400 tons. Here, the RNA mass is varied between 100 and 750 tons. For each variation, a design of the monopile is created with a first natural frequency of 0.29 Hz.
The results are given in terms of mass, pile diameter and soil penetration depth for water depths of 30m and 50m. These are projected against the current industry limits for the production of monopiles and hoisting capacity on installation vessels. Furthermore, it is shown above which prestress of the RNA mass the size of the support structure is significantly influenced. The combined results substantiate that the monopile will remain the choice support structure type in coming years and that RNA mass reduction leads to significant economic gain for wind farm developers. Additionally, a solution which offers further perspective to the application of the monopile is briefly discussed.